The present invention relates to an anodizing apparatus and method of anodizing a substrate to be processed in an electrolytic solution, and a porous substrate manufactured by the apparatus and method.
Porous silicon was found by A. Uhlir and D. R. Turner who were studying electropolishing of single-crystal silicon biased to a positive potential in an aqueous solution of hydrofluoric acid (to be abbreviated as HF hereinafter).
Later, to exploit the excellent reactivity of porous silicon, application to element isolation in which a thick insulating structure need be formed in manufacturing a silicon integrated circuit was examined, and a full isolation technology using a porous silicon oxide film (FIPOS: Full Isolation by Porous Oxidized Silicon) was developed (K. Imai, Solid State Electron 24, 159, 1981).
Recently, an applied technology to direct bonding in which a silicon epitaxial layer grown on a porous silicon substrate is bonded to an amorphous substrate or single-crystal silicon substrate via an oxide film has been developed (Japanese Patent Laid-Open No. 5-21338).
As another application example, porous silicon which emits light by itself has received a great deal of attention as a so-called photoluminescence or electroluminescence material (Japanese Patent Laid-Open No. 6-338631).
FIG. 5 is a view showing the arrangement of an anodizing apparatus for manufacturing porous silicon by anodizing a silicon substrate, disclosed in Japanese Patent Laid-Open No. 60-94737.
This anodizing apparatus is constituted by arranging anodizing tanks 1802a and 1802b consisting of Teflon (trade mark of du Pont in U.S.A) having an HF resistance to sandwich a silicon substrate 1801. The anodizing tanks 1802a and 1802b have platinum electrodes 1803a and 1803b, respectively.
The anodizing tanks 1802a and 1802b have grooves in side walls contacting the silicon substrate 1801, and 0-rings 1804a and 1804b of fluororubber are fitted in these grooves, respectively. The anodizing tanks 1802a and 1802b and the silicon substrate 1801 are sealed by the 0-rings 1804a and 1804b, respectively. The sealed anodizing tanks 1802a and 1802b are filled with HF solutions 1805a and 1805b, respectively.
In these anodizing tanks, the silicon substrate does not come into direct contact with the metal electrodes and is hardly contaminated by the metal electrodes. However, since the silicon substrate to be anodized is sealed by the 0-rings on its upper and lower surfaces, unanodized portions remain in the peripheral regions on the surfaces of the silicon substrate. In addition, since the silicon substrate to be processed itself is directly set and integrated with the anodizing tanks, the silicon substrate cannot be quickly exchanged.
To solve these problems, an anodizing apparatus which supports a silicon substrate at its side surface (beveling) regions has been developed (Japanese Patent Laid-Open No. 5-198556). This anodizing apparatus can anodize all surface regions of the substrate while preventing contamination from the metal electrode. Additionally, in this anodizing apparatus, since a wafer to be processed is fixed in a holder, and the holder is fixed in the anodizing tanks, i.e., the wafer is fixed in the anodizing tank in two steps, operability is largely improved as compared to the conventional apparatus which directly fixes a wafer in the anodizing tank to integrate the wafer with the anodizing tank.
Another anodizing apparatus has been developed, in which a silicon substrate is arranged between a silicon substrate to be processed and a metal electrode, and the electrode component from the metal electrode is shielded by the silicon substrate to prevent contamination due to the electrode component dissolved in the electrolytic solution (Japanese Patent Laid-Open No. 6-275598).
As still another method of preventing contamination from the electrode, a method of directly fixing the electrode to a silicon substrate to be processed via another silicon substrate has been developed (Japanese Patent Laid-Open No. 8-037173).
The anodizing apparatuses disclosed in Japanese Patent Laid-Open Nos. 5-198556, 6-275598, and 8-037173 are very practical because metal contamination rarely occurs, and all surface regions of a substrate can be anodized.
However, an anodizing apparatus having higher productivity is desired. For example, the silicon substrate arranged between the silicon substrate to be processed and the metal electrode is anodized. The silicon substrate must be discarded because it is in direct contact with the metal electrode or is contaminated by the metal component dissolved in the electrolytic solution, so the silicon material is wasted.
In the method of inserting the silicon substrate as an intermediate substrate between the electrode and the silicon substrate to be processed, the intermediate substrate is also dipped in the electrolytic solution and anodized. Hence, the intermediate substrate must be frequently exchanged, and the silicon material is wasted.